Secondary porosity in carbonate rocks, primarily related to vugs and fractures, impacts fluid flow and recovery efficiency in subterranean reservoirs. The porosity system may be complex in carbonates where the distribution of primary and secondary porosity varies from facies-to-facies at different scales. Rapid changes in carbonate depositional environments may create different facies within a short vertical scale. The subsequent diagenesis processes, such as dissolution, cementation, and dolomitization, may alter each facies differently. In carbonate rocks, the diagenesis process may create vugs, which are cavities in the rock that are visible to the unaided eye. The vugs may be categorized as isolated vugs or connected (touching) vugs. Tectonic stress may also superimpose fracture networks to the subterranean formation. For the pore spaces connected to fractures, solution-enhanced bedding planes and vugs (vug-to-vug) may enhance the fluid flow, and are well related to increased oil recovery rates. On the contrary, the porosity related to isolated (separate) vugs may contribute little to permeability, and the permeability may be controlled by the amount of interparticle pore space of the matrix. Thus, characterizing the different pore spaces in complex reservoirs can be a challenge.
The accuracy of the evaluation of complex reservoirs has improved since the introduction of borehole image logging and subsequent interpretation workflows. One such implementation regards a method to analyze image texture by delineating conductive and resistive heterogeneities. Such implementation, however, may suffer shortcomings attributable to heterogeneities that are larger than the image pad width that are not detectable (in the case of pad imaging tools), the limited classification of heterogeneities, and the lack of one or more links to one or more reservoir parameters. Another method entails characterizing complex reservoirs utilizing micro-electrical borehole images, such as computing the porosity distribution by applying Archie's law to each value of the conductivity image over short vertical windows, and then splitting the distribution into vuggy or matrix pore space by one or more cutoff methods. However, the distribution relies on the cutoff method, such that vuggy porosity and fracture porosity are evaluated separately. Such method also cannot characterize the position in space and the geometrical properties of the vuggy zones.